During the manufacture of steel sheets, the cold rolling process essentially creates two types of impurities on the sheet surface: first, surface carbon, which comes from the degradation of the rolling oils, and secondly, iron fines from the interactions with the cylinders used for rolling.
This surface pollution is problematic because it requires more frequent cleaning of the cylinders and the pickling baths are more quickly polluted. This obviously entails additional costs. The dirty sheets also have to be annealed longer, which is also more costly. Lastly, in the subsequent galvanization or painting steps, these depositions amount to adhesion flaws that have consequences on the corrosion resistance of the finished products.
To assess the surface cleanliness, there are two different methods that can be classified in two groups:                laboratory methods, which are precise but “off-line.” These methods are based on X-ray fluorescence, atomic absorption, mass spectrometry, etc. They generally require a long time and are costly to implement;        in-line control methods, which are generally quick but less precise. Among those, the “Scotch® Tape Test” (or Tesa-Test) is the best known method. It consists in applying on the sheet, in the most reproducible manner possible, a piece of adhesive, hereinafter called “Scotch® tape”, that will then be removed and stuck on white paper. The percentage of light reflected by the “Scotch® tape” charged with particles removed from the sheet is then measured. This is done either by comparison with standards, or using a specific device.        
This last method however depends on the operator, and in particular on the way that the Scotch® tape is applied on the sheet (application speed, pressure, removal speed, etc.). It results in a significant dispersion of the results, which can reach more than 20% on the reflectivity measurement.
Recently, a semi-automatic method was developed. This method allows to automatically apply the “Scotch® tape” on the sheet, which may be in motion, then to measure the reflectivity percentage, also done automatically. However, an operator is still present and the dispersion of the results is apparently only barely lower. Furthermore, the discontinuous nature of the measurements remains a major drawback (cf. CoilScooter-TG apparatus by the company INNSITEC Laser technologies GmbH—www.innsitec.com).
Even more recently, a completely automatic method based on the absorption of infrared radiation was studied. To our knowledge, it is still being developed and, in any case, is not widely spread (see Krauth P. J., “Contrôle de la propreté des surfaces d'acier”, La Revue de Métallurgie—CIT, June 2002).